Power conservation is important in many systems, such as but not limited to, wireless sensor networks (WSNs), which have low power requirements and low data rates. Long battery life (e.g., up to 10 years) is essential in such systems where line power is not available or if the system is mobile. However, in many applications, using and replacing batteries, even long-lived types, is impractical, involving such factors as hard-to-access locations, labor and replacement battery costs. Accordingly, solutions have been sought for harvesting or extracting electrical power from the environment.
One example of energy harvesting is that of harvesting power from ambient vibrations. Vibration harvesters can generate electricity from vibrations that are barely noticeable to the human touch. Commercially available vibration harvesters include the Energy Harvester made by Ferro Solutions. In an environment with vibrations at 28 Hz and 100 milli-G's, this harvester produces a power output of 9.3 mW. The power output scales linearly with increased vibration frequency and exponentially with increased g-force. When the electricity thus generated is not used immediately, it can be stored in a super capacitor. Another example is Continuum Control Corp.'s iPower energy harvesters, which can extract electric energy from mechanical vibrations, motion, or impact, and store it for use by wireless sensors or other electronic devices.
The patent literature also describes vibration harvesters. For example, U.S. Pat. No. 6,771,007 to Tanielian describes a piezoelectric device connected to a vibration source that converts vibration energy to electrical current. A plurality of pairs of oppositely polarized piezoelectric wafers deflect to produce an electrical current. Each pair of wafers are arranged back-to-back and electrically joined together. The wafers are connected to a set of micro-machined parts and form a cantilevered beam with a mass-weighted end whose movement is converted to electrical energy.
U.S. Pat. Nos. 5,934,882 to Olney and 6,407,484 to Oliver et al. also describe piezoelectric stacks and elements used as energy harvesters.
U.S. Pat. No. 6,433,465 to McKnight et al. describes an energy-harvesting device using electrostrictive polymers. The electrical response phenomenon of electrostrictive polymers is used to harvest electrical power from the movement of objects, e.g., human walking motion. The polymer material is incorporated into the soles of footwear and is either full- or partial-wave rectified and then if desired converted to a direct current (DC) voltage level for suitable battery charging and the like.